This invention relates to electrodes for measuring ion concentrations in aqueous solutions and to methods of manufacturing such electrodes and the ion-selective membranes they employ.
Ion-sensing electrodes selectively responsive to ionic activities in aqueous solutions are well known to the art. The known relationship between ionic activity and ion concentration permits these electrodes to be used to measure ion concentrations. In such electrodes, selective ion exchange occurs through an interface between a selective ion exchange material and the solution to be samples. Many recent electrode designs have employed ion-selective membranes which retain the ion-selective material in a matrix of organic material such as polyvinylchloride or other plastic.
Many prior art sensing membranes employing a plastic matrix are prepared by dissolving the ion-selective material in a solvent which either is, or has dissolved within it a plasticizer for the plastic. The plastic, in powdered form, is added to this mixture. The membrane is cast in its desired form by applying this mixture to a substrate having the desired shape and evaporating the solvent or otherwise curing the mixture. Examples of such electrodes are described in U.S. Pat. No. 3,450,631 issued to Bloch, et al, U.S. Pat. No. 3,635,212, issued to Watanabe, U.S. Pat. No. 4,271,002 issued to Hawkins and U.S. Pat. No. 4,242,191 issued to Schindler et al.
In some prior art electrodes, the substrate on which the ion-selective membrane is formed is incorporated as part of the electrode structure. In others, the membrane is removed from the substrate and attached to the body of the electrode. In such electrodes, sealing of the membrane to the remainder of the electrode structure has been problematic.
U.S. Pat. No. 4,233,136 issued to Spaziani, et al, addresses the problem of sealing the ion-selective membrane to the remainder of the electrode structure, in the context of incorporating an ion-selective membrane in the wall of a plastic tube. This method, like that of the prior art electrodes discussed above, involves dissolving powdered plastic in a volatile solvent containing a plasticizer and an ion-selective material and evaporating the solvent to form the membrane. In the Spaziani electrode, a plastic tube is provided with a lateral opening, and a cylindrical mandrel is inserted in the tube across the opening. The mixture is applied to the mandrel, filling the opening of the tube. The volatile solvent fuses the membrane to the plastic tube. After the membrane has formed, the mandrel is removed leaving the membrane in place.